1. Field of the Invention
The present invention relates to a system for optically and electronically ascertaining the existence and nature of precipitation and for measuring visibility in the. ambient air from a remote location.
2. Description of the Prior Art
A light emitting diode weather identification system has previously been developed to remotely detect the existence of precipitation and to determine the nature of precipitation. That is, the prior weather identification system can differentiate between rain and snow. This prior weather identification system was established with the intent of automating surface weather observations at airports. The ability to completely automate surface observations is critically dependent on the ability to automatically distinguish between rain and snow. This prior system is disclosed in U.S. Pat. No. 4,760,272 issued on Jul. 26, 1988.
The prior weather identification system is able to correctly identify different types of precipitation in a near field region by transmitting a partially coherent light source over a path length much shorter than that of prior optical precipitation systems. According to the system of U.S. Pat. No. 4,760,272 a partially coherent light source, such as an infrared light emitting diode, may be used in place of a laser source which conventional optical weather identification systems previously required. The partially coherent beam of light is transmitted over a short distance of less then one meter and is detected by an optical receiver located in communication with the transmitter and in spaced separation therefrom.
The percentage light intensity fluctuations (scintillation indices) detected by the optical receiver within certain frequency ranges are indicative both of the existence of precipitation and the nature of the detected precipitation. Frequency components above one kilohertz are indicative of rain. Snow induced frequencies are primarily at a few hundred hertz. Accordingly, electronic signals generated from the received scintillations are directed to at least two different band pass filters to quantify scintillations in low arid high frequency ranges. Scintillation signals in the low frequency range indicate snow while scintillations in the higher frequency range indicate rain. The low frequency band may be from twenty five hertz to two hundred fifty hertz, for example, while the high frequency band may extend from one kilohertz to four kilohertz.
Because high frequency components of rain drop induced scintillations are critical for discriminating between rain and snow, a half angle of incoherency which is too large may result in an insufficient number of usable signals in the high frequency band. Therefore, for all practical purposes the product of one half the angle of incoherency of the transmitter, as measured in radians, multiplied by the distance of the transmitter and receiver, must be no greater than about 2.5 millimeters.
While the system of U.S. Pat. No. 4,760,272 quite accurately determines the level and nature of precipitation, it does not provide an indication of visibility level. As a result of increasingly acute environmental problems, a need exists for accurate sensors to measure optical visibility in the atmosphere.
A number of prior visibility sensors have been devised and are available commercially. These conventional visibility sensors are of three general types. These are: (1) transmissometer instruments; (2) backscatter instruments, and (3) forwardscatter instruments. For more than thirty years the system used most widely to assess visibility has been the Federal Aviation Administration (FAA) runway visual range (RVR) transmissometer. This system uses a field sensor placed adjacent to the runway and measures the atmospheric transmission of a fixed seventy five meter path. A major problem with the RVR transmissometer, however, is its limited dynamic range. A well maintained transmissometer can only provide accurate visual ranges from one half its baseline to ten times its baseline (a factor of twenty). In addition, the absolute calibration for the transmissometer demands frequent field maintenance and may be contaminated by background light such as daylight and street lights. The lengthy baseline of seventy five meters that is required causes another problem for large scale field deployment. Due to these disadvantages, the use of a transmissometer as part of an automated weather station for unattended operation is impractical.
Backscatter visibility sensor instruments operate on the principle that the extinction coefficient may be related to the intensity of light scattered back from an object. Sources of error for these instruments arise from the uncertainty of the empirical relationship between the back scattering coefficient and visibility, and from the failure to allow for optical extinction between the instrument and the scattering volume. However, the major problem for field operation of such backscatter instruments is the contamination of reflected signals by rain and snow. These ambiguities prevent backscatter visibility sensors from providing reliable measurements of visibility during precipitation.
Forwardscatter instruments operate on the assumption that light scattered at certain angles in the forward direction can be related to the extinction coefficient. Forwardscatter instruments are less sensitive to precipitation contamination than backscatter instruments. Forwardscatter instruments also have a much larger dynamic range, on the order of a factor of several thousand, as contrasted with the dynamic range of the transmissometer, which is twenty.
A major problem with the present commercially available forwardscatter visibility sensors is that calibration depends on the light source intensity. Therefore, a constant monitor for the output of light intensity is required. Dust on the optical system, which cannot be avoided for unattended field operation, is another source of error. Moreover, the large angle of forward scattering caused by the irregular shapes of snow also contaminates the visibility measurements. Furthermore, forwardscatter instruments for measuring visibility have not previously been practical due to the uncertainty of the relationship between the scattering of light and the extinction coefficient.